1. Field of the Invention
The present invention relates to methods and apparatus for fabricating optical devices, such as optical integrated circuits (ICs).
2. Description of the Background Art
Communications systems utilizing optical components (e.g., splitters, routers, couplers, filters, etc.) are being used to address bandwidth issues in the communications industry. Such optical components may be fabricated as planar optical waveguide structures. FIG. 1 is a cross section of an exemplary waveguide structure 100. Planar optical waveguide structures are formed on a planar surface of a substrate 102 and typically comprise a core 104 surrounded by one or more cladding layers 106, 108. The core material has a higher index of refraction relative to the one or more layers of cladding material, to optically confine a light beam propagating within the optical waveguide. Currently, separate materials are being used to form each of the core 104, lower cladding 106, and the upper cladding 108.
Fabrication of planar optical components on silicon and silica substrates currently exists. These fabrication methods advantageously seek to use processing equipment traditionally used in integrated circuit (IC) fabrication. Currently, the size, shape and degree of integration of optical devices and integrated circuits on a common substrate are constrained by the size and shape of the substrate. Further, unlike IC designers who commonly utilize both vertical and horizontal structures in IC device design and fabrication, optical devices are generally constrained to pathways travelling in a single plane with stringent constraints on pathway curvature. As a result of these constraints, optical device layouts and fabrication methods favor die shapes having elongated rectangular dimensions.
For integrated optical device system fabrication, the number of devices that can be fabricated on a single substrate is limited by the size of the devices that can be formed on a circular substrate as well as device interconnections required for coupling them together. Because of the size limitation of the optical devices, often a circuit comprising multiple optical devices must be formed on one or more substrates, and be externally connected together by optical fibers in order to form the desired optical system. The use of external optical fibers to couple the optical devices increases optical losses and reliability of optical systems utilizing them, rendering less than satisfactory performance of the circuit.
A problem with silicon substrates, besides the circular shape, is it must be isolated from the waveguide to avoid interfering with the light wave traveling down the waveguide. Light waves traveling in a waveguide comprise two orthogonally polarized modes. For waveguide applications, one polarization is horizontal to the substrate and the other polarization is orthogonal to the substrate. If the lower cladding is too thin, the two orthogonal modes see a different effective refractive index resulting in birefringence, a consequential dispersion phenomenon that would limit the width of the transmission window. In order to minimize the effect of birefringence on optical devices formed on silicon substrates, a relatively thick, e.g., about 15 μm to about 30 μm, lower cladding is needed to serve as a buffer layer.
A conventional waveguide structure requires at least three deposition steps and one mask level. For example, the lower cladding layer must first be deposited to isolate the substrate from the waveguide. Next, a core layer is deposited and patterned to form the waveguide paths. An upper cladding layer is then deposited thereover. The upper cladding layer must be thick enough to prevent interference from external ambient light, i.e., light from the environment outside the device. In addition, each of these layers may, and currently do, require post deposition heat treatment to obtain the desired optical properties.
Another problem with silicon substrates in optical device fabrication results because the substrate and the material layers comprising the waveguide structure each have a different coefficient of thermal expansion (CTE). During fabrication, the substrate and the material layers of the waveguide structure are exposed to several heat cycles. These heat cycles as well as the different CTEs may cause the material layers of the waveguide structure to shrink more than the substrate, undesirably bowing the substrate. This also introduces stress in the films. This effect increases as larger and larger silicon substrates are used.